BACKGROUND: In patients with diarrhoea, faecal elastase-1 is used to detect exocrine pancreatic insufficiency. Diarrhoea is defined as >85% stool water content. METHODS: We analysed elastase-1 in 519 stool samples from 310 patients unprocessed as well as after lyophilization in a standard laboratory lyophilizator. Stool water content was calculated by weight difference before and after lyophilization. RESULTS: 151 stool samples were classified as 'diarrhoea' (mean stool water content 88.8%); all others had a mean water content of 75.4%. In the 'diarrhoea' samples, elastase-1 levels were considerably higher after lyophilization--with a calculated normal water content of 75% (606 +/- 359 microg/g, mean +/- s)--compared to measurement when unprocessed (279 +/- 151 microg/g; P < 0.0001). In 16 of the 151 cases (11%), the abnormally low elastase-1 level below 200 microg/g found in unprocessed stools proved normal (>200 microg/g) after lyophilization and correction for stool water content. CONCLUSION: Diarrhoea can result in falsely decreased elastase-1 levels in a number of patients with non-pancreatogenic diarrhoea. Lyophilization is a simple measure by which to determine elastase-1 independently of stool water content. Lyophilization of stool samples can therefore help to prevent wrong positive elastase-1 test results.